In order to support higher data rate and spectrum efficiency, new wireless technologies have been introduced. For example, the third generation partnership project (3GPP) long term evolution (LTE) system has been introduced into 3GPP Release 8 (R8).
The LTE downlink (DL) transmission is based on orthogonal frequency division multiple access (OFDMA) air interface, and the LTE uplink (UL) transmission is based on single-carrier (SC) DFT-spread OFDMA (DFT-S-OFDMA). The use of single-carrier transmission in the UL is motivated by the lower peak-to-average power ratio (PAPR) compared to multi-carrier transmission such as orthogonal frequency division multiplex (OFDM). For flexible deployment, the 3GPP R8 LTE systems support scalable transmission bandwidths of either 1.4, 2.5, 5, 10, 15 or 20 MHz. The R8 LTE system may operate in either frequency division duplex (FDD), time division duplex (TDD) or half-duplex FDD modes.
In the R8 LTE system, each radio frame (10 ms) consists of 10 equally sized sub-frames of 1 ms. Each sub-frame consists of 2 equally sized timeslots of 0.5 ms each. There may be either 7 or 6 OFDM symbols per timeslot. 7 symbols per timeslot are used with a normal cyclic prefix, and 6 symbols per timeslot are used with an extended cyclic prefix. The sub-carrier spacing for the R8 LTE system is 15 kHz. An alternative reduced sub-carrier spacing mode using 7.5 kHz is also possible. A resource element (RE) corresponds to one (1) sub-carrier during one (1) OFDM symbol interval. 12 consecutive sub-carriers during a 0.5 ms timeslot constitute one (1) resource block (RB). Therefore, with 7 symbols per timeslot, each RB consists of 12×7=84 REs. A DL carrier may comprise scalable number of RBs, ranging from a minimum of 6 RBs up to a maximum of 110 RBs. This corresponds to an overall scalable transmission bandwidth of roughly 1 MHz up to 20 MHz. Normally, a set of common transmission bandwidths is specified, (e.g., 1.4, 3, 5, 10, or 20 MHz). The basic time-domain unit for dynamic scheduling in LTE is one sub-frame consisting of two consecutive timeslots. Certain sub-carriers on some OFDM symbols are allocated to carry pilot signals in the time-frequency grid.
In the R8 LTE DL direction, a wireless transmit/receive unit (WTRU) may be allocated by the evolved Node-B (eNB) to receive its data anywhere across the whole LTE transmission bandwidth. In the R8 LTE UL direction, a WTRU may transmit on a limited, yet contiguous set of assigned sub-carriers in an FDMA arrangement. This is called single carrier (SC) FDMA. For example, if the overall OFDM signal or system bandwidth in the UL is composed of sub-carriers numbered 1 to 100, a first WTRU may be assigned to transmit its own signal on sub-carriers 1-12, a second WTRU may transmit on sub-carriers 13-24, and so on. An eNB would receive the composite UL signals across the entire transmission bandwidth normally from a plurality of WTRUs at the same time, but each WTRU would transmit in a subset of the available transmission bandwidth. Frequency hopping may be applied in UL transmissions by a WTRU.
In order to further improve achievable throughput and coverage of LTE-based radio access systems, and in order to meet the IMT-Advanced requirements of 1 Gbps and 500 Mbps in the DL and UL directions, respectively, LTE-Advanced (LTE-A) is currently under study in 3GPP standardization body. One major improvement proposed for LTE-A is the carrier aggregation and support of flexible bandwidth arrangement. It would allow DL and UL transmission bandwidths to exceed 20 MHz in R8 LTE, (e.g., 40 MHz), and allow for more flexible usage of the available paired carriers. For example, whereas R8 LTE is limited to operate in symmetrical and paired FDD mode, (e.g., DL and UL are both 10 MHz or 20 MHz transmission bandwidth each), LTE-A would be able to operate in asymmetric configurations, for example DL 10 MHz paired with UL 5 MHz. In addition, composite aggregate transmission bandwidths may be possible with LTE-A, (e.g., DL a first 20 MHz component carrier+a second 10 MHz component carrier paired with an UL 20 MHz component carrier). The composite aggregate transmission bandwidths may not necessarily be contiguous in frequency domain. Alternatively, operation in contiguous aggregate transmission bandwidths may also be possible, (e.g., a first DL component carrier (CC) of 15 MHz is aggregated with another 15 MHz DL component carrier and paired with a UL component carrier of 20 MHz).
FIG. 1A shows discontinuous spectrum aggregation and FIGS. 1B and 1C show continuous spectrum aggregation. The LTE R8 UL transmission format uses DFT-S OFDM using a DFT precoder. The DFT precoder may be applied to the aggregate bandwidth, (i.e., across all the component carriers), in case the bandwidths are contiguous, as shown in FIG. 1B. Alternatively, the DFT precoder may be applied per component carrier, (e.g., up to 110 RBs or 20 MHz maximum), as shown in FIG. 1C.
FIGS. 2A and 2B show an intra-mobility management entity (MME)/serving gateway handover procedure in LTE R8. In LTE R8, hard handover is used and handover procedure is restricted to one carrier, (i.e., one component carrier).
An eNB is provided with a WTRU context including information regarding roaming restrictions either at connection establishment or at the last tracking area (TA) update (step 102). The source eNB configures the WTRU measurement procedures according to the area restriction information (step 104). Measurements provided by the source eNB may assist the function controlling the WTRU's connection mobility.
The WTRU gets uplink allocation for transmission of a measurement report, which is triggered by the rules set by, for example, system information, specification, etc. (step 106), and transmits a measurement report to the source eNB once triggered (step 108).
The source eNB makes a handover decision based on the measurement report and radio resource management (RRM) information (step 110). The source eNB issues a handover request message to the target eNB at step 112 passing the necessary information to prepare the handover at the target eNB including WTRU X2 signaling context reference at the source eNB, WTRU S1 EPC signaling context reference, target cell identity (ID), KeNB, RRC context including the cell radio network temporary identity (C-RNTI) of the WTRU in the source eNB, access stratum (AS)-configuration, EUTRAN radio access bearer (E-RAB) context and physical layer ID of the source cell+medium access control (MAC) for possible radio link failure (RLF) recovery), or the like. The WTRU X2/WTRU S1 signaling references enable the target eNB to address the source eNB and the evolved packet core (EPC). The E-RAB context includes necessary radio network layer (RNL) and transport network layer (TNL) addressing information, and quality of service (QoS) profiles of the E-RABs.
The target eNB may perform the admission control dependent on the received E-RAB QoS information to increase the likelihood of a successful handover, if the resources may be granted by target eNB (step 114). The target eNB configures the required resources according to the received E-RAB QoS information and reserves a C-RNTI and optionally a random access channel (RACH) preamble. The AS-configuration to be used in the target cell may either be specified independently (i.e., an “establishment”) or as a delta compared to the AS-configuration used in the source cell (i.e., a “reconfiguration”).
The target eNB prepares handover with layer 1 and layer 2 and sends a handover request acknowledgement to the source eNB (step 116). The handover request acknowledgement message includes a transparent container to be sent to the WTRU as an RRC message to perform the handover. The container includes a new C-RNTI, and target eNB security algorithm identifiers for the selected security algorithms. The container may optionally include a dedicated RACH preamble, and some other parameters, for example access parameters, system information blocks (SIBs), etc. The handover request acknowledgement message may also include RNL/TNL information for the forwarding tunnels, if necessary. As soon as the source eNB receives the handover request acknowledgment message, or as soon as the transmission of the handover command is initiated in the downlink, data forwarding may be initiated.
The source eNB generates an RRC message, (i.e., RRCConnectionReconfiguration message including the mobilityControlInformation towards the WTRU), and sends the RRC message to the WTRU (step 118). The WTRU receives the RRCConnectionReconfiguration message with the necessary parameters (i.e., new C-RNTI, target eNB security algorithm identifiers, and optionally dedicated RACH preamble, target eNB SIBs, etc.) and detaches from the source cell and synchronizes to the target cell (step 120).
The source eNB delivers buffered and in-transit packets to the target eNB (step 122), and sends the SN STATUS TRANSFER message to the target eNB to convey the uplink packet data convergence protocol (PDCP) sequence number (SN) receiver status and the downlink PDCP SN transmitter status of E-RABs for which PDCP status preservation applies (i.e., for radio link control (RLC) acknowledged mode (AM)) (step 124). The uplink PDCP SN receiver status includes at least the PDCP SN of the first missing UL service data unit (SDU) and may include a bit map of the receive status of the out of sequence UL SDUs that the WTRU needs to retransmit in the target cell, if there are any such SDUs. The downlink PDCP SN transmitter status indicates the next PDCP SN that the target eNB may assign to new SDUs, not having a PDCP SN yet. The source eNB may omit sending this message if none of the E-RABs of the WTRU may be treated with PDCP status preservation.
After receiving the RRCConnectionReconfiguration message including the mobilityControlInformation, the WTRU performs synchronization to the target eNB and accesses the target cell via RACH following a contention-free procedure if a dedicated RACH preamble was allocated in the handover command or following a contention-based procedure if no dedicated preamble was allocated (step 126).
The target eNB responds with UL allocation and timing advance (step 128). When the WTRU has successfully accessed the target cell, the WTRU sends the RRCConnectionReconfigurationComplete message (C-RNTI) to confirm the handover along with an uplink buffer status report to the target eNB to indicate that the handover procedure is completed for the WTRU (step 130). The target eNB verifies the C-RNTI sent in the handover confirm message. The target eNB can now begin sending data to the WTRU.
The target eNB sends a path switch message to the MME to inform that the WTRU has changed a cell (step 132). The MME sends a user plane update request message to the serving gateway (step 134). The serving gateway switches the downlink data path to the target side, and sends one or more “end marker” packets on the old path to the source eNB and then may release any U-plane/TNL resources towards the source eNB (step 136).
The serving gateway sends a user plane update response message to the MME (step 138). The MME confirms the path switch message with the path switch acknowledgment message (step 140). By sending the WTRU context release message, the target eNB informs success of handover to the source eNB and triggers the release of resources (step 142). Upon reception of the WTRU context release message, the source eNB may release radio and C-plane related resources associated to the WTRU context (step 144). Data packet is then transmitted via the target eNB.
In the above conventional LTE R8 handover procedure, the measurements currently defined to support LTE Rel-8 handover are not sufficient to support handover of an aggregation of component carriers in LTE-A since a single carrier is implicitly assumed in Rel8. In addition, handover of an entire carrier aggregation would be problematic. For example, the relative quality of each component carrier may not necessarily be the same from each cell and so the best handover time for each component carrier would not be simultaneous.